Experimental conditions

There will be a driving force for a phase transformation if the conditions of a system are changed in such a way that the system moves from one phase field into another in the phase diagram. In this chapter we shall examine the character of such phase transformations and we shall find that they depend upon the experimental method of controlling and changing the conditions. It is important first to realize that the possibility of efficiently controlling the various state variables is very different. For gaseous and liquid phases it is comparatively easy to control the pressure. It can be kept constant or it can be changed gradually according to an experimental programme. At any moment it is very uniform in the system apart from effects due to the surface energy of curved phase interfaces. For solid systems it is more difficult to control the pressure, in particular during a phase transformation resulting in a volume change. This may give rise to local deformation and internal stresses. On the other hand, solid phases are usually so dense and rigid that the thermodynamic effect of pressure differences and stresses can often be ignored. From a practical point of view we may often regard the pressure as an experimental variable which can be reasonably well maintained at a low enough level to have a negligible effect.